Technical Abstract:
In an earlier study by Boulton et al. (1991) a genetic determinant of virulence of the Maize streak mastrevirus (MSV) was mapped to the large intergenic promoter region (LIR) of a MSV isolate from Nigeria. For this isolate a single nucleotide change in the promoter of the viral replication initiator protein gene from TATA to TGTA resulted in a change from a severe to mild symptom phenotype. To test the hypothesis that the MSV LIRs of other isolates also contain the determinant of virulence, we cloned and sequenced the genome of a mild [MSV-KL (mild)] and a severe [MSV-Km (severe)] isolate from Kenya. The two clones contained an identical TATA sequence in the LIR promoter but differed by thirteen nucleotides in other portions of the LIR. To test whether these nucleotide differences also affected MSV symptom severity, we constructed recombinants MSV-KL (m)[LIR (Km(s)] and MSV-Km(s) [LIR (KL (m)] by exchanging Nsi1-BamH1 (nts. 2187-2689) fragments between the wild type clones. When the susceptible maize hybrid P3379 was infected with either chimeric clone, an intermediate phenotype was displayed, suggesting that the LIR in conjunction with another genomic region or regions determine symptom severity.